It is known to use feedtubes or standpipes as a preferred means for interconnecting an ink storage compartment of an ink jet pen with a thin film printhead (including orifice plate) which is mounted on one surface of the pen body housing. These feedtubes or standpipes serve to accommodate the necessary capillary draw of ink from the ink reservoir and through the firing chamber of the printhead to the print orifii thereof.
One example of such a feedtube is disclosed in U.S. Pat. No. 4,771,295 issued to Baker et al and assigned to the present assignee. Another example of such a feedtube is disclosed in U.S. Pat. No. 4,794,409 issued to Cowger et al and also assigned to the present assignee. Both of these U.S. Patents are incorporated fully herein by reference.
In recent experiments, it has been observed that standpipes which have a maximum diameter of the portion of the standpipe in which air accumulates below a certain minimum value will cause the ink jet pen to be greatly susceptible to depriming. In these so-called small standpipe ink jet pens, air bubbles that form in the standpipe, and particularly those that form and accumulate at the ink entrance or input opening of the standpipe, block the flow of ink to the ink jet printhead mounted at the other end of the standpipe to thus starve the printhead of ink and cause the pen to deprime. The printhead is simply incapable of generating enough pressure to pull ink past the air bubble in the standpipe, and the air bubble at the ink input opening of the small diameter standpipe seals off the flow of ink at this opening and causes the pen to deprime.
Historically, small diameter standpipes have been designed so that during pen priming, when there are relatively high flow rates of ink through the standpipe, high ink flow velocities are obtained through the standpipe and bubbles of air should theoretically be swept away. In practice, however, this rarely happens for the reason given above.